1. Field of the Invention
The present invention relates to a mercury-free arc tube for a discharge lamp having a closed chamber portion opening portions on both ends of which are sealed, in which Na or Sc halides are sealed together with a rare gas and the electrode rods are provided to oppose to each other, and which has an internal volume of 50 μl.
2. Description of the Related Art
FIG. 11 shows a discharge lamp in the related art. The discharge lamp has such a structure that a front end portion of a quartz-glass arc tube 5 is supported with one lead support 2 protruded forward from an insulating base 1, a rear end portion of the arc tube 5 is supported with a concave portion 1a of the base 1, and the arc tube 5 is sustained at a portion near its rear end with a metal supporting member 4 fixed to a front surface of the insulating base 1. A front end-side lead wire 8 led from the arc tube 5 is fixed to the lead support 2 by welding, while a rear end-side lead wire 8 is passed through a bottom wall 1b constituting the concave portion 1a of the base 1 and secured to a terminal 3 provided to the bottom wall 1b by welding. A symbol G denotes a cylindrical ultraviolet shielding globe made of the glass to cut off an ultraviolet component in a bandwidth that is harmful to the human body from the light that is emitted from the arc tube 5. This ultraviolet shielding globe G is deposited integrally to the arc tube 5.
Then, the arc tube 5 has such a structure that a closed glass globe 5a in which electrode rods 6, 6 are provided between a pair of front and rear pinch sealed portions 5b, 5b to oppose to each other and into which luminous substances (Na and Sc halides and Hg) are sealed together with a rare gas is formed. A molybdenum foil 7 for connecting the electrode rod 6 protruded into the closed glass globe 5a and the lead wire 8 led from the pinch sealed portion 5b is sealed in the pinch sealed portion 5b, and thus an air tightness in the pinch sealed portion 5b is maintained.
That is, a tungsten electrode rod that is excellent in a heat resistance and durability is most desirable as the electrode rod 6. However, such tungsten has a coefficient of linear expansion that is largely different from that of the quartz glass constituting the arc tube, and gets to badly fit the quartz glass, and is inferior in the air tightness to the quartz glass. For this reason, the air tightness in the pinch sealed portion 5b is secured by connecting a molybdenum foil 7 to the tungsten electrode rod 6 and then sealing the molybdenum foil 7 with the pinch sealed portion 5b. The molybdenum foil 7 is excellent in elasticity and flexibility and also, it shows relatively well fitting property to the quartz glass.
However, a difference in temperature in the pinch sealed portion 5b becomes large when the arc tube 5 is turned ON and OFF. Thus, when the arc tube 5 is turned ON, a thermal stress is generated between the electrode rod and the quartz glass layer, coefficients of linear expansion of which are largely different. In particular, because the recent arc tube is constructed to make the instantaneous lighting possible, a rate of temperature rise is large and the thermal stress is generated abruptly. Then, such a problem existed that, if such situation is repeated many times, a crack appears in the pinch sealed portion which corresponds a quartz glass layer) 5b that seals the electrode rod 6 and the sealed substance leaks out, which result in the lightening failure or the shorter lifetime.
With regard to this problem, the structure shown in Japanese Patent Unexamined Publication No. JP-A-2001-15067 which is described below was proposed based on such a conclusion. The conclusion is that, in a case that a residual compressive strain caused in the pinch sealed portion 5b in the course of the arc tube manufacturing still remains over predetermined areas, because the thermal stress generated in the quartz glass layer in the pinch sealed portion can be scattered due to the temperature rise when the arc tube is turned ON. Accordingly, the crack is hard to appear in the quartz glass layer in the pinch sealed portion and therefore, the lifetime of the arc tube is extended.
In other words, as shown in FIG. 12, the JP-A-2001-15067 shows such a structure that a residual compressive strain layer 9 is formed on adhesive surfaces which is defined between the quartz glass layer and the electrode rod 6 in the pinch sealed portion 5b over a predetermined wide area. Since the thermal stress generated on a boundary between the electrode rod 6 and the quartz glass layer is absorbed and scattered by the residual compressive strain layer 9 and transmitted to the quartz glass layer side, the crack that leads a leakage of the sealed substance is not generated in the quartz glass layer in the pinch sealed portion 5b. 
Also, this Hg sealed in the closed glass globe 5a is a very useful buffer substance to relieve the damage of the electrode by maintaining a predetermined tube voltage and reducing an amount of collision of the electron to the electrode. However, such Hg is an environmentally hazardous material. For this reason, recently the development of the so-called mercury-free arc tube into which Hg acting as the environmentally hazardous material is not sealed is accelerated.
When the mercury-free arc tube is employed, the tube voltage is lowered and the tube power necessary for a discharge cannot be achieved. Therefore, an electric current supplied to the arc tube, which is called tube current, must be increased to increase the tube power, and a load to the electrode is increased correspondingly. As a result, such a problem has arisen that the electrode is damaged which is represented by occurring (consumption or darkened, and either reduction in the luminous efficiency or disappearance of the arc is brought about. To deal with this problem, it may solve the problem by enlarging the diameter of the electrode rod 6. However, if the diameter of the electrode rod 6 is enhanced too thick, a difference in a thermal compression amount between the electrode rod and the quartz glass layer appears largely in cooling the pinch sealed portion after the pinch sealing is applied. Then separation of the boundary between the quartz glass layer and the electrode rod is caused. Thus, the residual compressive strain layer 9 having a size enough to absorb and relieve the thermal stress, which is generated when the arc tube is turned ON, cannot be formed in the quartz glass layer of the pinch sealed portion 5b around the electrode rod 6. Such anew problem has arisen that the crack that leads a leakage of the sealed substance is generated in the pinch sealed portion 5b when the arc tube is turned ON and OFF.